As one of the most successful industry standard, MPEG-2 has been widely used in digital video and entertainment applications for over 10 years. As the result, the majority of video contents, such as DVD and digital broadcasts, are encoded in MPEG-2. However, the newer H.264/AVC video standard typically improves coding efficiency by a factor of two over MPEG-2. When AVC is used to compress video sequences, a user can store more contents in the same storage amount and enjoy high quality video streaming with smaller bandwidth. Thus, it is desirable to transform an MPEG-2 bit stream to an H.264/AVC video bit stream. The most straightforward approach for such transcoding is to decode an MPEG-2 bit stream into raw data (YUV) and use an AVC encoder to compress the raw data. However, this approach does not utilize any additional picture information obtained during the MPEG-2 decoding processes.
In addition, the resulting bit stream may not be optimal in terms of picture quality. In the AVC standard, coding mode selection plays an important role in improving the compression efficiency and quality. For example, the AVC standard allows adaptive picture mode selection when encoding an interlaced video sequence so a frame can be coded as a single frame using frame coding, or a frame can be divided into two fields and coded as two individual fields using field coding. Using a properly selected picture mode during transcoding has been shown to result in a 2 db improvement over either frame only coding or field only coding. However, the prior art transcoding methods use an exhaustive search-based two pass scheme. Each frame is first encoded by frame mode, and the corresponding bit rate and distortion are recorded. The same frame is then encoded by field mode, and the corresponding bit rate and distortion are recorded. A cost function is used to compare the two coding modes, including their corresponding bit rates and distortions. The mode with smaller cost is selected. Although good performance can be obtained using the two pass scheme, its complexity renders its use in real life applications prohibitive.